1. Field of the Invention
The present invention generally relates to an image forming apparatus such as a copier, a printer, or a facsimile machine using an electrophotographic process, and a process cartridge used in the image forming apparatus.
2. Description of the Related Art
In these days, miniaturization, high image quality, and ecology are key factors in developing image forming apparatuses. A “cleaner-less” image forming apparatus is one solution for reducing the size of an image forming apparatus. In a cleaner-less image forming apparatus, a developing unit functions also as a cleaning unit for removing toner remaining on a photoconductor after transferring a toner image, and thus a dedicated cleaning device is omitted.
For example, patent document 1 discloses a cleaner-less image forming apparatus including a developing unit that forms a toner image by developing a latent image formed on a uniformly-charged image carrier with a developer including spherical toner particles and carrier particles and held on a developer carrier. The developing unit contains yellow toner, cyan toner, magenta toner, or black toner, and also functions as a cleaning unit that reclaims toner remaining on the image carrier after transferring a toner image onto a recording medium (hereafter such remaining toner may be called post-transfer residual toner) and while developing another toner image. The developing/cleaning unit constitutes an image forming unit, and multiple image forming units are disposed along a conveyor belt in the disclosed cleaner-less image forming apparatus. A recording medium is conveyed by the conveyor belt through the image forming units to form a color image. In the disclosed cleaner-less image forming apparatus, an area of a spherical toner particle covered by a sizing agent is limited to 15% to 50% of the entire surface area, the shape factor SF-1 of the spherical toner particles is between 100 and 140, and the shape factor SF-2 is between 100 and 120.
In a color image forming apparatus including multiple image forming units as described above, toner images of different colors are formed by the image forming units, transferred to and superposed on an intermediate transfer body in sequence by a primary transfer member, and then transferred together onto a recording medium by a secondary transfer member. One problem in employing a cleaner-less configuration in such a color image forming apparatus is that if toner on the intermediate transfer body (or an image receiving body) is transferred back onto a photoconductive drum (hereafter such toner may be called reverse-transferred toner) and enters a developing unit, toners of different colors are mixed in the developing unit and cause a color shift.
Patent document 2 discloses an image forming apparatus that tries to solve this problem. The disclosed image forming apparatus includes a photoconductor on which a toner image is formed, and an intermediate transfer body onto which the toner image is transferred from the photoconductor. The toner image on the intermediate transfer body is transferred and fused to a recording medium to form an image on the recording medium. The disclosed image forming apparatus also includes a cleaning device comprising a scraper for scraping post-transfer residual toner on the photoconductor and a housing containing the scraper and having an opening toward the surface of the intermediate transfer body.
In the image forming apparatus disclosed in patent document 2, the cleaning device prevents mixing of toners of different colors and prevents a charging unit from being tainted. However, providing a cleaning device with such a housing increases the size and costs of an image forming apparatus.
Patent document 3 discloses a contact charging method used in an electrophotographic device. In the disclosed method, a bias with a polarity opposite to the charge polarity of a photoconductor is applied to at least one of multiple charging units, a bias of the same polarity as the charge polarity of the photoconductor is applied to at least one of the remaining charging units, and the photoconductor is charged to a predetermined potential by the charging units. With this method, however, since a photoconductor is charged by multiple charging units, the photoconductor is exposed to electrostatic hazards at two or more positions. As a result, the influence of corona products on the photoconductor increases, and the electrostatic characteristics of the photoconductor are gradually degraded as it is charged and discharged repeatedly. With the disclosed contact charging method, this problem becomes especially prominent because a photoconductor is first charged to a polarity opposite to a desired charge polarity and then charged to the desired charge polarity.
Meanwhile, in a color laser printer, toner images of different colors are formed on a photoconductive drum or an image carrier using toners or developers of corresponding colors, and the toner images are electrically transferred to and superposed on an image receiving body such as a recording medium or an intermediate transfer belt to form a multi-color toner image.
Also, in some color laser printers, a photoconductive drum is provided for each of multiple colors. Such color laser printers are called tandem color laser printers.
In a color-image forming process, when a toner image of one color is transferred onto an image receiving body on which a toner image of another color is already present, the charge amount of the already present toner image increases. Increase in the charge amount increases the chance of the toner of the already present toner image to be reverse-transferred onto the photoconductive drum from the image receiving body. When toner is reverse-transferred from an image receiving body onto a photoconductive drum and sticks to a charging roller or enters a developing unit, it causes problems such as potential irregularities and degrades image quality. To prevent such problems, conventional color laser printers include a cleaning device for reclaiming residual toner on a photoconductive drum.
Meanwhile, as described above, a “cleaner-less” method or configuration (see, for example, patent document 1) makes it possible to reduce the size of an image forming apparatus, to reduce waste toner, and to reduce toner consumption per page.
The “cleaner-less” method or configuration is also applicable to a color image forming apparatus using an intermediate transfer body.
In a color image forming apparatus, toner images of different colors are formed by multiple image forming units, transferred to and superposed on an intermediate transfer body in sequence, and then transferred together onto a recording medium by a secondary transfer member. One problem in employing a cleaner-less configuration in such a color image forming apparatus is that if toner on the intermediate transfer body (or an image receiving body) is transferred back onto a photoconductive drum and enters a developing unit, toners of different colors are mixed in the developing unit and cause a color shift.
Patent document 4 tries to solve the above problem. An image forming apparatus disclosed in patent document 4 is configured to measure the amount of reverse-transferred toner on a developing roller based on the operating time of the image forming apparatus, the areas of images formed, and the charge amount of a developer. When the measured amount of reverse-transferred toner reaches a predetermined limit, the disclosed image forming apparatus removes reverse-transferred toner on the developing roller either by forming a solid image on a photoconductive drum and transferring the solid image onto a recording medium, a conveyor belt, or an intermediate transfer belt; or by separating the photoconductive drum and the developing roller and then rotating the developing roller alone.
However, it is generally difficult to correctly measure the amount of reverse-transferred toner since it is influenced by the aging of toner. Also, in a cleaner-less image forming apparatus where a developing unit reclaims residual toner on a photoconductive drum after transferring a toner image and while developing another toner image, if toner from an upstream developing unit is mixed with toner in a downstream developing unit, it causes a color shift in the downstream developing unit.
[Patent document 1] Japanese Patent No. 3248047
[Patent document 2] Japanese Patent Application Publication No. 2003-186364
[Patent document 3] Japanese Patent Application Publication No. 4-310980
[Patent document 4] Japanese Patent Application Publication No. 2005-316371